An atmospheric pressure ionization interface is used in order to ionize and perform mass analysis of a liquid sample or of analysis target components in a solution separated into components by a liquid chromatograph. Known typical atmospheric pressure ionization methods include electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI). Generally, this sort of atmospheric pressure ionization interface is often used in combination with a quadrupole mass spectrometer, ion trap mass spectrometer or time-of-flight mass spectrometer.
An atmospheric pressure ionization interface, particularly an ESI interface, has the characteristic of readily generating polyvalent ions having multiple charges in the process of ionizing a compound. Polyvalent ions, depending on the valence, have a smaller mass-charge ratio m/z than the molecular weight of the original compound, and thus have the advantage of making it possible to relatively lower the mass-charge ratio range of ions constituting the target of analysis. In particular, when analyzing high molecular weight compounds such as proteins and peptides, the mass-charge ratio of a monovalent ion may exceed the measurable range of a mass spectrometer, and using polyvalent ions makes it possible to keep the mass-charge ratio within the measurable range of the mass spectrometer. Due to this fact, mass analysis using polyvalent ions is highly effective for identifying high molecular weight compounds.
When a high molecular weight compound is ionized with an ESI ion source to perform mass analysis, peaks derived from ions with various valences appear on the mass spectrum (for example, see FIG. 1 of Non-patent Literature 2). When computational processing is performed by a technique called deconvolution on this sort of mass spectrum in which multiple polyvalent ion peaks are observed, a neutral mass spectrum is determined (for example, see FIG. 2 of Non-patent Literature 2), and based on this, the molecular weight of the target compound is obtained (see Patent Literature 1, Non-patent Literature 1, etc.).
The above-described conventional technique using deconvolution is useful when multiple (normally, about 10 or more) polyvalent ion peaks with different valences derived from the target compound are observed on the mass spectrum, and make it possible to determine molecular weight with adequate certainty. However, when only two or three polyvalent ion peaks are present on the mass spectrum, a technique using deconvolution cannot be said to be particularly effective. The reason for this is that, assuming M is the molecular weight corresponding to a polyvalent ion on the mass spectrum, i.e. the molecular weight of the compound, n is the valence of the ion, H is the molecular weight of a proton, and m is the observed ion mass/charge ratio m/z, if only two to three peaks corresponding to the following formula (1) are present, there is the possibility that those peaks will not be derived from the target compound but will rather be accidental matches.M=n(m+H)  (1)Furthermore, especially when the intensity of some of the ion peaks is low, it is difficult to distinguish them from noise peaks, etc., and it is rather difficult to judge based on the mass spectrum whether or not those multiple ion peaks are derived from the same target compound.